Carbon nanotubes (CNTs) are self-assembling nanostructures comprised of graphite sheets rolled up into cylinders [Iijima, Nature, 1991, 354, 56-58]. Such nanostructures are termed single-walled carbon nanotubes (SWNTs) if they are comprised of a single cylindrical tube [Iijima et al., Nature 1993, 363, 603-605; and Bethune et al., Nature 1993, 363, 605-607]. CNTs comprising two or more concentric tubes are termed double-walled carbon nanotubes (DWNTs) and multiwall carbon nanotubes (MWNTs), respectively. Regarding SWNTs, the diameter of these species can typically range from 0.4 nm to ca. 3 nm, and the length from ca. 10 nm to centimeters.
CNTs have found use in a wide variety of applications including conductive and high-strength composites, electrode materials for high capacity batteries, efficient field emission displays and radiation sources, and functional nanoscale devices [Baughman et al., Science, 2002, 297, 787-792]. However, the primary barriers to their widespread use remain the high costs involved in their synthesis and purification.
SWNTs are currently produced in a variety of ways, including arc discharge, laser oven, and chemical vapor deposition (CVD). While efforts are being made to scale up the production of these materials, all currently known synthesis methods result in large amounts of impurities in the product. For example, carbon-coated metal residues typically comprise 20-30 wt % of HiPco-produced CNT material [Nikolaev et al., Chemical Physics Letters, 1999, 313, 91-97], and ca. 60% non-nanotube carbon is formed in the arc discharge process. Both the metal and carbonaceous impurities can compromise the performance of carbon nanotubes in many applications.
Most current processes for purifying CNTs rely on one or both of the following steps: (1) removing the carbon coating that encapsulates the metal catalysts via oxidation with O2, CO2, or H2O vapor, at temperatures of 300-800° C. [e.g., Chiang et al., J. Phys. Chem. B 2001, 105, 1157-1161; Chiang et al., J. Phys. Chem. B 2001, 105, 8297-8301], or by wet chemical oxidation with oxidants including nitric acid, H2O2, or KMnO4, during which sonication is frequently employed [e.g., Liu et al., Science, 1998, 280, 1253-1256]; and (2) using centrifugation or filtration to separate the CNTs from the soluble impurities [e.g., Bandow et al., J. Phys. Chem. B, 1997, 101, 8839-8842]. These aggressive processes typically result in severe damage to, and loss of, CNTs. Additionally, the processes are often limited to small batch yields, low yields, and/or low purity. Consequently, an efficient industrial scale purification process to remove these impurities is essential, as many of the applications of CNTs require highly-purified CNTs.